Abstract: The physical process of diffraction by planar gratings'is analysed,based on the generalized second-order coupled-wave theory. Chief items discussed include. 1) an analysis of the structure of the light field iu a planar gratiug, 2) the relation between modes and coupling, 3) a method of measuring the degree of coupling between different orders of light field, 4) the relation between the Bragg conditions and the degeneration conditions, 5) energy transference between differert orders of light field inside a grating and 6) the influence of boundary parameters of a grating on the diffraction process.